SQL Parsing Flow Diagram

Subject: SQL Parsing Flow Diagram
Doc ID: Note:32895.1 Type: REFERENCE
Last Revision Date: 20-OCT-2005 Status: PUBLISHED


PURPOSE
~~~~~~~
The document gives a overview of Parsing.

SCOPE & APPLICATION
~~~~~~~~~~~~~~~~~~~
For users requiring a general overview of how parsing works.


Introduction
============

This article show the parsing mechanism as a flow diagram.
Its main purpose is to show the difference between a 'soft' and a
'hard' parse.

It is intended to give a feel of how parsing operates to make
explanation of parsing activity easier.


SQL Parsing Flow Diagram
========================

Statement
Submitted
|
Is it in an open cursor?--------------YES----V
| |
NO |
| |
Is SESSION_CACHED_CURSORS = Value |
and cursor in --------------YES----V In these 3 cases we
Session Cursor cache? | know that the cursor has
| | already been parsed, so
NO | re-parsing is
| | unnecessary.
Is HOLD_CURSOR=Y |
and cursor in --------------YES----V
Held cursor cache? |
| |
NO |
| | ^
OPEN A CURSOR | CLIENT SIDE |
| | -------------|
Statement is Hashed and compared | SERVER SIDE |
with the Hashed value in the sql area | V
| V
Is it in sql area? --YES-(Soft Parse)--> ---------
| | |
NO | EXECUTE |
| | |
PARSE STATEMENT ('Hard' Parse)---------> ---------


NOTES
=====

1. A cursor is an address on the client that points to the memory
location of a SQL statement on the server.
Multiple-client cursors may point at the same address on the server.

2. Remember that 'Client' and 'Server' sides may reside on the same
machine - in which case Client/Server is a logical distinction.

3. If a cursor is open, then the statement will be in the sql_area,
so no parsing is necessary.

This is why locks may remain when a client is terminated
abnormally (such as a PC Client being turned off without
closing open cursors).

4. SESSION_CACHED_CURSORS is the initialisation parameter that
specifies how many cursors to hold open for a particular session.

5. HOLD_CURSOR is an precompiler parameter that specifies that an
individual cursor should be held open.

See Page 11-3 of the Programmer's guide to the Oracle Precompilers.

6. Both the soft and hard parse register as a parse in tkprof. Hashing the
current statement updates the parse count.

7. Soft parse avoids many of the steps taken during the parse phase for a
particular statement. Initial syntactic and semantic checks are made and
then the statement is hashed and compared with hashed statements in
the SQL area. If a match is found, then existing information is used
and relatively expensive steps (such as query optimization etc.) are avoided.

8. The 10053 event is only invoked during a hard parse.


Additional Search Words
~~~~~~~~~~~~~~~~~~~~~~~

HARD; HOLD_CURSOR; PARSE; SESSION_CACHED_CURSORS; SOFT; SQL;
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